Ionization detector for gas chromatography

ABSTRACT

AN IONIZATION DETECTOR IN WHICH THE COLLECTING ELECTRODE IS PLACED BETWEEN CHARGED BARRIER ELECTRODES. THE BARRIER ELECTRODES SET UP A POTENTIAL BARRIER AROUND THE COLLECTOR ELECTRODE. CHARGED PARTICLES HAVING ENERGIES HIGH ENOUGH TO SURMOUNT THE POTENTIAL BARRIER FALL INTO THE POTENTIAL WELL BETWEEN THE BARRIER ELECTRODES AND ARE EFFICIENTLY COLLECTED BY THE COLLECTOR ELECTRODE. THE BIAS VOLTAGE ON THE BARRIER ELECTRODES CAN BE ADJUSTED SO THAT ONLY CHARGED PARTICLES OF A DESIRED ENERGY WILL BE COLLECTED. THE DETECTOR CAN BE MADE TO RESPOND SELECTIVELY TO ONLY A DESIRED CLASS OF COMPOUNDS IN THE CHROMATOGRAPHIC COLUMN EFFLUENT BY SUITABLE ADJUSTMENT OF THE BIAS VOLTAGE ON THE BARRIER ELECTRODES.

June 1971 M. E. scbLNlcK IONIZATION DETECTOR FOR GAS CHROMATOGRAPHY Filed April 6, 1967 2 Sheets-Sheet l 3 60 ATTORNEY PHOS.

INVFNTOR TlN E SCOLNICK MAR PHOS.

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Patented June 15, 1971 3,585,003 IONIZATION DETECTOR FOR GAS CHROMATOGRAPHY Martin E. Scolnick Kensington, Calif., assignor to Varian Associates, Palo Alto, Calif.

Filed Apr. 6, 1967, Ser. No. 628,925

Int. Cl. G01n 31/12 US. Cl. 23-254 7 Claims ABSTRACT OF THE DISCLOSURE An ionization detector in which the collecting electrode is placed between charged barrier electrodes. The barrier electrodes set up a potential barirer around the collector electrode. Charged particles having energies high enough to surmount the potential barrier fall into the potential well between the barrier electrodes and are efliciently collected by the collector electrode. The bias voltage on the barrier electrodes can be adjusted so that only charged particles of a desired energy will be collected. The detector can be made to respond selectively to only a desired class of compounds in the chromatographic column eflluent by suitable adjustment of the bias voltage on the barrier electrodes.

BACKGROUND OF THE INVENTION This invention relates generally to ionization detectors for use in gas chromatography. In particular, the invention relates to an ionization detector which responds selectively to a specific class of compounds present in the efiluent gas from a chromatographic column.

Ionization detectors used in gas chromatography consist of a flow chamber enclosing an energy source and a collector electrode. Efiluent gas from the chromatographic column flows through the chamber and is partially ionized by the energy source. The ions and electrons thus produced are collected by the collector electrode, and the resulting electrical current is amplified and recorded by external electronic apparatus. When pure carrier gas flows through the detector chamber the rate of ion collection is essentially constant, thus resulting in a constant external current. However, when a mixture of solute vapor (sample component) and carrier gas flows through the detector chamber the rate of ion collection is a function of time rather than a constant. The plot of the external current as a function of time results in the familiar chromatogram consisting of a series of peaksone peak for each com ponent origianlly present in the sample.

- With the exception of the electron capture detector, ionization detectors respond to a wide range of solute molecules. For example, the flame ionization detector (FID) responds to all organic molecules. However, for certain analysis, it is desirable that the detector respond only to a specific class of molecules. For example, in pesticide residue analyses, the most useful detector is one which responds only to the pesticide residue and is nonresponsive to other molecules in the sample mixture. In amino acid analyses a. detector responsive solely to nitrogen containing molecules considerably simplifies the analysis.

SUMMARY OF INVENTION The present invention provides an ionization detector which can be adjusted to respond selectively to a specific class of molecules. The detector comprises a source of ionizing energy and a collector electrode disposed between electrically charged barrier electrodes. The barrier electrodes set up a potential barrier around the collector electrode, and only those charged particles having energies high enough to surmount the potential barrier are collected by the collector electrode. The bias voltage on the barrier electrodes is adjusted so that only charged particles in a specific energy range are collected. In this manner the detector responds selectively to a desired class of molecules in the eflluent gas from the column.

In one test of the inventive concept, a conventional flame ionization detector was modified by placing a barrier electrode, in the form of a wire loop, on each side of the collector electrode. The collector electrode was at ground potential and the potential on the barrier electrodes was varied. A mixture of C hydrocarbon and a phosphorus-containing compound was injected into the gas chromatograph, and the effect of varying the barrier potential was observed. As the barrier potential was increased, the detectors response to the hydrocarbon decreased. Eventually, at a barrier potential of 4 volts, the hydrocarbon response was completely suppressed and only the phosphorus peak remained on the recorder trace. Thus, at this particular barrier voltage the detector was responsive solely to phosphorus-containing molecules.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described with reference to the accompanying drawings, in which;

FIG. 1 is a gas chromatographic system incorporating the present ionization detector,

FIG. 2 is a plot of electric potential as a function of axial distance in the detector shown in FIG. 1,

FIG. 3 is a chromatogram of a mixture of a C hydrocarbon and a phosphorus-containing compound showing the eflect of barrier electrode bias voltage on the suppression of hydrocarbon response,

FIG. 4 is an alternative coaxial electrode embodiment of the detector shown in FIG. 1,

FIG. 5 is an alternative plane parallel electrode embodiment of the detector shown in FIG. 1,

FIG. 6 is an alternative spherical electrode embodiment of the detector shown in FIG. 1, and

FIG. 7 is a barrier potential detector wherein the source of ionizing energy is a radioactive isotope.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a chromatographic system incorporating a flame ionization detector modified in accordance with the present invention. A sample mixture to be analyzed is injected into the system via an injection port 11 and vaporized into a stream of carrier gas provided by a compressed gas source 12 and a pressure regulator 13. The vaporized sample mixture is swept by the carrier gas into a chromatographic column 14 wherein the various components in the mixture separate from one another. The eflluent stream from column 14 (consisting of carrier gas and separated sample components) enters a detector 16 via an effluent input channel 17 drilled into detector base 18. Air and hydrogen also flow into detector 16 via respective input channels 19 and 21, and hydrogen channel 21 communicates with etfluent channel 17 to form a central flow passage 22. The mixture of column efiluent gas and hydrogen flows through passage 22 and is ignited at the tip of jet 23 to form a flame 24. Air channel 19 terminates in a diffuser 26 which promotes smooth air flow and uniform burning of flame 24. The detector is enclosed by a cylindrical housing 27 and a loose-fitting lid 28. Lower and upper barrier electrodes, 29 and 31, respectively, are disposed on each side of a collector electrode 32, and electrical connections to the electrodes are made via feed-through insulators 33. Barrier electrodes 29 and 31 are connected externally to the slider 34 of a potentiometer resistor 36 across whose fixed terminals is applied a voltage from battery 37. Collector electrode 32 is connected externally to the input of an electrometer amplifier 38 whose output drives a recorder 39.

Molecules of the separated sample components are ionized in flame 24 to form positive and negative charged particles (positive and negative molecular ions plus electrons). Barrier electrodes 29 and 31 are biased negatively with respect to collector electrode 32 and jet 23 (collector electrode 32 is at virtual ground potential through the input of amplifier 38). Therefore, the positive charged particles are attracted to the negatively charged barrier electrodes where they are neutralized on contact. However, the negative charged particles are repelled by the barrier electrodes and, in effect, must climb uphill against an energy barrier to reach collector electrode 32. The shape of this energy barrier is shown in FIG. 2, which is a plot of electric potential (for a negative charge) as a function of distance along a central axis perpendicular to the planes of electrodes 29, 31 and 32. As shown in FIG. 2, the voltage on lower electrode 29 creates a potential barrier whose height is directly proportional to the applied voltage. Negative particles which surmount the potential barrier fall into the potential well centered on electrode 32, and are collected and amplified to form the detector signal current.

The ability of the detector to respond selectively to, or to discriminate between, various molecular species may depend upon several mechanisms. For example, the ionization of a molecule containing phosphorus atoms may produce charged particles of higher energy than the charged particles produced from the ionization of a hydrocarbon molecule. The height of the potential barrier can then be adjusted so that only the higher energy phosphorus particles surmount the barrier and are collected. The lower energy hydrocarbon particles cannot surmount the barrier and hence are not collected. With the potential barrier so adjusted the detector would respond selectively to molecules containing phosphorus atoms and would have no response to hydrocarbons.

Another mechanism explaining the selective response of the detector is that the combustion of sample molecules may physically increase the size (height and/or width) of flame 24. Different molecular species may increase the flame size by varying degrees, e.g., hydrocarbons increase flame height by while phosphorus containing molecules increase flame height by 50%. Thus, in the case of phosphorus compounds, negative particles are released at a point closer to collector electrode 32. This corresponds to being released at a point further up on the slope of the potential barrier. Hence such negative particles spend less of their energy in overcoming the lower portion of the barrier, and are thus able to surmount the barrier and be collected. Negative particles from hydrocarbon molecules are released further down the slope of the potential barrier (because of the relatively small increase in flame height) and are unable to surmount the barrier and be collected.

Still another mechanism for explaining the detectors selective response is that the combustion of certain molecules may increase the temperature of flame 24, thereby decreasing the density of the surrounding gas. The corresponding increase in the mean free path of released negative particles allows them to surmount the potential barrier and be collected at electrode 32. Charged particles released in a combustion reaction which produces a relatively small increase in flame temperature cannot diffuse across the potential barrier as easily as particles produced in a high temperature flame. This is because the higher temperature increases the agitational velocity of the particles and decreases the gas density. The combination of these two effects increases the particle diffusion rate across the potential barrier. Once again, the potential barrier can be adjusted in height to exploit this dif ference in flame temperature so as to provide a selective response to a desired class of molecules.

FIG. 3 shows how the detectors response can be altered by varying the voltage on barrier electrodes 29 and 31. A mixture of a phosphorus containing pesticide and a C hydrocarbon was injected into the apparatus of FIG. 1 maintained at the following conditions:

Detector temp.=200 C. H flow rate=30 mL/min. N flow rate=30 mL/min. Air flow rate=265 mL/min. Column temp.=190 C.

As the voltage on barrier electrodes 29 and 31 was made more negative, the ratio of pesticide (labeled phos. in FIG. 3) to C peak heights increased. At a barrier voltage of 4.0 volts the peak height ratio was infinite, i.e., the C peak was completely masked out. The -4.0 barrier voltage is given solely by way of example, and it will be apparent that the barrier voltage required to suppress a given chromatographic peak will depend largely on the geometry of the individual detector.

The detector shown in FIG. 1 employs barrier and collector electrodes in the form of wire loops, however alternative electrode configurations can be employed, as shown in FIGS. 4, 5 and 6. Referring to FIG. 4, there are shown electrodes 29a, 31a and 32a in the form of coaxial cylinders constructed of metallic mesh. These cylindrical electrodes function in a manner entirely analogous to the corresponding loop electrodes shown in 'FIG. 1. Similarly,

I FIG. 5 shows electrodes 2%, 31b and 32b in the form of parallel plates of metallic mesh, and FIG. 6 shows electrodes 29c, 31c and 320 in the form of concentric spherical shells. The alternative electrode configurations shown in FIGS. 4, 5 and 6 are all functionally equivalent to the loop configuration of FIG. 1.

Although the invention has been described with particular reference to a modified flame ionization detector, the inventive concept of an adjustable barrier potential surrounding the collector electrode can be applied to any detector wherein the efl luent gas from a chromatographic column is ionized by an energy source and the resulting charged particles are collected to form a signal current. Such detectors are discussed in a paper by J. E. Lovelock in Analytical Chemistry, 33, 162-177 (February 1961). In the detector shown in FIG. 1 the energy source is a hydrogen flame. However, the other energy sources, such as radioactive isotopes, can also be employed to ionize the eflluent gas from the chromatographic column. Rererring to FIG. 7, there is shown a barrier potential detector in which the source of ionizing energy is a radioactive isotope. =Effluent gas from a chromatographic column enters the detector chamber 41 through an inlet 42 and leaves the detector through an outlet 43. Component molecules in the eflluent gas are ionized by a radioactive isotope in the form of a metallic foil 44 disposed within chamber 41. Negative particles formed during ionization of the component molecules are collected by a collector electrode 32d disposed between barrier electrodes 29d and 31d. The functioning of the remainder of the apparatus is similar to that described previously for the detector in FIG. 1. 1

The detectors in FIGS. 1 and 7 are shown with the collector and barrier electrodes biased for the collection of negative particles. However, it will be apparent to thoes skilled in the detector art that the polarity of the bias voltages on the electrodes can be reversed so that the current due to positive particles will be recorded.

What is claimed is:

1. A gas chromatographic system for separating the individual components in a mixture and detecting only a desired class of said separated components, comprising in combination: a chromatographic column; means for flowing a carrier gas through said column; means for injecting a mixture of components into said column, whereby the carrier gas sweeps said mixture through the column causing the components in the mixture to separate from one another; detector means disposed in the flow path of said separated components, said detector comprising, an energy source capable of ionizing molecules of said separated components, a collector electrode to collect charged particles of a selected polarity produced as the result of the ionization of said component molecules, another electrode, and means or producing a fixed D.C. potential barrier of selectable magnitude between said other electrode and said collector electrode, the magnitude of said potential barrier being adjusted such that only those charged particles produced by the ionization of a selected class of separated components are collected by said collector electrode; and means for recording the electrical current corresponding to the collection of said charged particles.

2. A gas chromatographic detector responsive to molecules of separated mixture components flowing therethrough, comprising in combination: a flame capable of ionizing molecules of said separated components; a collector electrode to collect charged particles of a selected polarity produced as the result of the ionization of said component molecules, another electrode; means for producing a fixed DC. potential barrier of selectable magnitude between said other electrode and said collector electrode, the magnitude of said potential barrier being adjusted such that only those charged particles produced by the ionization of a selected class of separated components are collected by said collector electrode; and means for recording the electrical current corresponding to the collection of said charged particles.

3. The detectorv according to claim 2, wherein said means for producing a barrier potential includes at least one potential barrier electrode disposed between said other electrode and said collector electrode, and means for applying a selectable fixed D.C. electrical voltage to said barrier electrode.

4. The detector according to claim 3', wherein said barrier electrode is disposed in plane parallel spaced relation with said collector electrode and said other electrode.

5. The detector according to claim 3, wherein said barrier electrode is disposed in cylindrical coaxial spaced relation with said collector electrode.

6. The detector according to claim 3, wherein said barrier electrode is disposed in concentric spherical spaced relation with said collector electrode.

7. A gas chromatographic detector responsive to molecules of separated mixture components flowing there through comprising in combination: a detector base incorporating flow passages for air, hydrogen, and elfiuent gas from a chromatographic column, said hydrogen and column effiuent flow passages communicating with one another inside said base to form a combined flow passage; a burner =jet mounted on said base such that a central flow passage within said jet communicates with the combined flow passage in said base; first and second potential barrier electrodes disposed above and adjacent said burner jet; a collector electrode disposed between said first and second barrier electrodes, said burner jet comprising another electrode; means connected to said first and second barrier electrodes for applying a fixed D.C. electrical voltage of selectable magnitude and polarity thereto; and means connected to said collector electrode for recording the electrical current flowing therethrough.

References Cited UNITED STATES PATENTS 3,009,098 11/ 196 1 Simons, Jr 250-43.5R 3,095,278 6/1963 Green, Jr 23-254E 3,129,062 4/1964 Ongkiehong et al. 23-232C 3,175,886 3/1965 Krzeminiski et al. 23254E MORRIS O. WOLK, Primary Examiner R. M. REESE, Assistant Examiner U.S. Cl. X.R. 23232; 250-44 

